(1) Field of the Invention
This invention generally relates to amperometry, i.e. the apparative and operative means for quantitative electroanalytical determination of the concentration of an electroactive species of interest in a given sample and specifically to an amperometric cell of the type known as Clark cell and disclosed, e.g. in U.S. Pat. No. 2,913,386, as well as to amperometric methods using such cells. The term "electroactive species" as used herein generally refers to a substance, or substance component, that is capable of being oxidized or reduced by a suitably charged electrode.
While many modifications of the Clark cells are known, they share the following apparatus features:
(1) a sensing electrode which may be cathodic or anodic;
(2) a corresponding counter electrode;
(3) an aqueous electrolyte in contact with both the sensing electrode and the counter electrode; and
(4) a membrane that separates the aqueous electrolyte from the sample (termed to be "external" to the cell) and is essentially impermeable to the aqueous electrolyte yet permeable to the electroactive species of interest (also termed EASI).
Amperometric cells having the above features (1) to (4) are also termed MEACs (Membrane-Enclosed Amperometric Cells) herein. The sample external to the MEAC and containing the electroactive species generally is a fluid and may be a gas or a liquid in contact with the side or surface of the membrane opposite to that contacted by the electrolyte.
The common operative means of electroanalysis with MEACs include the steps of:
providing an amperometric cell having a sensing electrode, a counter electrode, an aqueous electrolyte in contact with the sensing electrode and the counter electrode, and a membrane that is substantially impermeable to the electrolyte but permeable to the EASI for containing the electrolyte within the cell and for separating it from the fluid maintained external to the cell;
applying a predetermined potential across the sensing electrode and the counter electrode;
measuring a cell current generated by reaction of an electroactive species with the sensing electrode; and
deriving a signal from such current that is indicative of the concentration of the EASI in the fluid sample.
(2) Description of the Prior Art
While there is no theoretical limit to the applicability of these amperometric means to the determination of concentrations of any particular electroactive species, considerable practical difficulties have been encountered when attempting to use commercial MEACs for uses other than oxygen measurements. For example, the above mentioned U.S. Patent to Clark, in addition to disclosing an oxygen monitoring method that is eminently suitable for practical purposes, sets forth that various other and either electro-reducible or electro-oxidizable species can be measured. However, practical implementation of amperometric determination of hydrogen concentration, for example, proved to be very difficult, c.f. U.S. Pat. No. 4,563,249, while measuring strongly oxidizing substances, such as ozone, seemed to call for a radical deviation from accepted MEAC structures, e.g using rotating electrodes as suggested in U.S. Pat. No. 3,960,673, or to require non-steady state operating conditions, or to use completely different systems, such as disclosed in U.S. Pat. No. 4,409,183. The performance of commercially available amperometric cells for measuring ozone concentrations leaves much to be desired as will be explained in more detail below, notably with regard to very long stabilization periods that would not be acceptable for routine monitoring purposes of the type required in modern manufacturing or processing plants.
As a matter of practice ozone (O.sub.3) can be regarded as a "strong" oxidizing agent which, when measured on a normal hydrogen electrode scale at pH=0 and 25.degree. C., generally exhibits a potential of at least about+1.3 Volts and which tends to show "slow stabilization" and "poor reproducibility" when analyzed by conventional methods with MEACs.
"Slow stabilization" means that an unduly long period of time is required for the response of a MEAC to a step change of the concentration of the electroactive species of interest (also referred to herein as EASI), e.g. upon sudden elimination of the EASI from the fluid that is external to the MEAC. While such response is asymptotic as a matter of principle, as a matter of practice a "reasonable" approximation to stability is expected to occur with a commercial MEAC within a couple of minutes, say typically in less than 10 minutes and preferably within 5 minutes, in a system suitable for routine measurements.
However, when using a commercially available MEAC for ozone detection, stabilization may require periods of up to 3 hours and this, obviously, is not suitable for quantitative measurement intended for routine operation in manufacturing or processing plants.
As regards an "acceptable degree" of reproducibility of measurement, this again is a matter of practice rather than theory; in commercial amperometry, however, one would normally expect a degree of reproducibility of "at least" 10% meaning a maximum deviation of repeated measurements under identical conditions of not more than 10% and typically better than 5%, i.e. .+-.5% deviation and preferably .+-.1% deviation. So far and according to tests made in the course of research leading to the present invention, such a degree of reproducibility cannot generally be achieved in routine operations with commercially available ozone measuring devices regardless of their working principle; this is evidenced by the fact that there is an acute need for commercial ozone measurement means which would satisfy these requirements.
In sum, amperometric routine measurements with commercially available MEACs tend to have one or both of the above defects when the EASI is ozone and it can be said with confidence that conventional electroanalytical methods are not generally suitable for effective and efficient quantitative determination of ozone.
Further, no simple yet reliable calibration methods are available for measurement of highly reactive substances, notably when the latter are such as typically ozone.